.\" $OpenBSD: X509_STORE_CTX_set_verify_cb.3,v 1.12 2023/05/30 07:37:34 op Exp $
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.Dd $Mdocdate: May 30 2023 $
.Dt X509_STORE_CTX_SET_VERIFY_CB 3
.Os
.Sh NAME
.Nm X509_STORE_CTX_verify_cb ,
.Nm X509_STORE_CTX_set_verify_cb ,
.Nm X509_STORE_CTX_get_verify_cb
.Nd set and retrieve verification callback
.Sh SYNOPSIS
.In openssl/x509_vfy.h
.Ft typedef int
.Fo (*X509_STORE_CTX_verify_cb)
.Fa "int ok"
.Fa "X509_STORE_CTX *ctx"
.Fc
.Ft void
.Fo X509_STORE_CTX_set_verify_cb
.Fa "X509_STORE_CTX *ctx"
.Fa "X509_STORE_CTX_verify_cb verify_cb"
.Fc
.Ft X509_STORE_CTX_verify_cb
.Fo X509_STORE_CTX_get_verify_cb
.Fa "X509_STORE_CTX *ctx"
.Fc
.Sh DESCRIPTION
.Fn X509_STORE_CTX_set_verify_cb
sets the verification callback of
.Fa ctx
to
.Fa verify_cb
overwriting any existing callback.
.Pp
The verification callback can be used to modify the operation of
certificate verification, either by overriding error conditions or
logging errors for debugging purposes.
The use of a verification callback is not essential, and should not
be used in security sensitive programs.
.Pp
Do not use this function.
It is extremely fragile and unpredictable.
This callback exposes implementation details of certificate verification,
which change as the library evolves.
Attempting to use it for security checks can introduce vulnerabilities if
making incorrect assumptions about when the callback is called.
Additionally, overriding
.Fa ok
may leave
.Fa ctx
in an inconsistent state and break invariants.
.Pp
Instead, customize certificate verification by configuring options on the
.Vt X509_STORE_CTX
before verification, or applying additional checks after
.Xr X509_verify_cert 3
completes successfully.
.Pp
The
.Fa ok
parameter to the callback indicates the value the callback should return
to retain the default behaviour.
If it is zero then an error condition is indicated.
If it is 1 then no error occurred.
As the default behaviour is internal to the verifier, and possibly unknown
to the caller, changing this parameter is inherently dangerous and should not
normally be done except for debugging purposes, and should not be expected to
be consistent if the verifier changes.
If the flag
.Dv X509_V_FLAG_NOTIFY_POLICY
is set, then
.Fa ok
is set to 2 to indicate the policy checking is complete.
.Pp
The
.Fa ctx
parameter to the callback is the
.Vt X509_STORE_CTX
structure that is performing the verification operation.
A callback can examine this structure and receive additional information
about the error, for example by calling
.Xr X509_STORE_CTX_get_current_cert 3 .
Additional application data can be passed to the callback via the
.Sy ex_data
mechanism.
.Pp
The verification callback can be set and inherited from the parent
structure performing the operation.
In some cases (such as S/MIME verification) the
.Vt X509_STORE_CTX
structure is created and destroyed internally and the only way to set a
custom verification callback is by inheriting it from the associated
.Vt X509_STORE .
.Sh RETURN VALUES
.Fn X509_STORE_CTX_get_verify_cb
returns a pointer to the current callback function
used by the specified
.Fa ctx .
If no callback was set using
.Fn X509_STORE_CTX_set_verify_cb ,
that is a pointer to a built-in static function
which does nothing except returning the
.Fa ok
argument passed to it.
.Sh EXAMPLES
Default callback operation:
.Bd -literal
int
verify_callback(int ok, X509_STORE_CTX *ctx)
{
	return ok;
}
.Ed
.Pp
This is likely the only safe callback to use.
.Pp
Simple and terrible example that should not be used.
Suppose a certificate in the chain is expired and we
wish to continue after this error:
.Bd -literal
int
verify_callback(int ok, X509_STORE_CTX *ctx)
{
	/* Tolerate certificate expiration */
	if (X509_STORE_CTX_get_error(ctx) == X509_V_ERR_CERT_HAS_EXPIRED)
		return 1;
	/* Otherwise don't override */
	return ok;
}
.Ed
.Pp
While this example is presented for historical purposes,
this is not the correct way to accomplish this.
The verification flag
.Dv X509_V_FLAG_NO_CHECK_TIME
should be set on the
.Vt STORE_CTX
using
.Xr X509_VERIFY_PARAM_set_flags 3
instead.
.Pp
Full featured debugging logging callback - note that the output and
order that things happen from this can change over time and should not
be parsed or expected to be consistent.
In this case the
.Fa bio_err
is assumed to be a global logging
.Vt BIO ,
an alternative would to store a
.Vt BIO
in
.Fa ctx
using
.Sy ex_data .
.Bd -literal
int
verify_callback(int ok, X509_STORE_CTX *ctx)
{
	X509 *err_cert;
	int err,depth;

	err_cert = X509_STORE_CTX_get_current_cert(ctx);
	err =	X509_STORE_CTX_get_error(ctx);
	depth =	X509_STORE_CTX_get_error_depth(ctx);

	BIO_printf(bio_err,"depth=%d ",depth);
	if (err_cert) {
		X509_NAME_print_ex(bio_err,
		    X509_get_subject_name(err_cert), 0,
		    XN_FLAG_ONELINE);
		BIO_puts(bio_err, "\en");
	} else
		BIO_puts(bio_err, "<no cert>\en");
	if (!ok)
		BIO_printf(bio_err, "verify error:num=%d:%s\en",
		    err, X509_verify_cert_error_string(err));
	switch (err) {
	case X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT:
		BIO_puts(bio_err, "issuer= ");
		X509_NAME_print_ex(bio_err,
		    X509_get_issuer_name(err_cert), 0,
		    XN_FLAG_ONELINE);
		BIO_puts(bio_err, "\en");
		break;
	case X509_V_ERR_CERT_NOT_YET_VALID:
	case X509_V_ERR_ERROR_IN_CERT_NOT_BEFORE_FIELD:
		BIO_printf(bio_err, "notBefore=");
		ASN1_TIME_print(bio_err,
		    X509_get_notBefore(err_cert));
		BIO_printf(bio_err, "\en");
		break;
	case X509_V_ERR_CERT_HAS_EXPIRED:
	case X509_V_ERR_ERROR_IN_CERT_NOT_AFTER_FIELD:
		BIO_printf(bio_err, "notAfter=");
		ASN1_TIME_print(bio_err, X509_get_notAfter(err_cert));
		BIO_printf(bio_err, "\en");
		break;
	case X509_V_ERR_NO_EXPLICIT_POLICY:
		policies_print(bio_err, ctx);
		break;
	}
	if (err == X509_V_OK && ok == 2)
		/* print out policies */

	BIO_printf(bio_err,"verify return:%d\en",ok);
	return(ok);
}
.Ed
.Sh SEE ALSO
.Xr X509_STORE_CTX_get_error 3 ,
.Xr X509_STORE_CTX_get_ex_new_index 3 ,
.Xr X509_STORE_CTX_new 3 ,
.Xr X509_STORE_CTX_set_error 3 ,
.Xr X509_STORE_CTX_set_flags 3 ,
.Xr X509_STORE_CTX_set_verify 3 ,
.Xr X509_STORE_set_verify_cb 3 ,
.Xr X509_verify_cert 3 ,
.Xr X509_VERIFY_PARAM_set_flags 3
.Sh HISTORY
.Fn X509_STORE_CTX_set_verify_cb
first appeared in OpenSSL 0.9.6c and has been available since
.Ox 3.2 .
.Pp
.Fn X509_STORE_CTX_get_verify_cb
first appeared in OpenSSL 1.1.0 and has been available since
.Ox 7.1 .
.Pp
.Fn X509_STORE_CTX_verify_cb
first appeared in OpenSSL 1.1.0 and has been available since
.Ox 7.2 .
.Sh CAVEATS
In general a verification callback should
.Sy NOT
return a changed value of
.Fa ok
because this can allow the verification to appear to succeed
in an unpredictable way.
This can effectively remove all security from the application because
untrusted or invalid certificates may be accepted.
Doing this can possibly make
.Xr X509_verify_cert 3
return what appears to be a validated chain of certificates that has not
been validated or even had the signatures checked.
